A conventional form of turbine rotor has a drum with a T-shaped channel for receiving the complementary T-shaped roots of a multiplicity of turbine blades. The other ends of the turbine blades are formed with integral shrouds which together form a shroud ring when assembled. Steam turbine moving blades require precision profiles that are most economically machined individually and then attached in rows to the turbine rotor.
Such T-shaped roots have parallelogram shapes which allow the “T” on the root to rotate into the rotor root slot or channel. To enable access of the last blade, a circumferential gap between each root is provided. This allows the blade roots to be closed up to create a larger gap for the last blade. The blade tip shrouds have no gaps. The access gap here is provided by the twisting of the blade, so that the parallelogram-shaped shroud rotates and becomes more circumferentially compact. After assembly of the last blade, the root gaps have to be filled with T-shaped shims, and the tips untwist until they become circumferentially in contact. The last few shims are in halves, and the last shim is held in place by a caulking material.
Thus existing configurations of T-shaped roots require the use of shims to secure them in place. The problem with this is that production costs are high, due partly to the need for skilled operatives and partly due to the complexity and cost of the shims themselves. The complexity of the shim shapes, causes them to be costly to produce in small numbers, and to require a high degree of assembly skill.
Other fastening types which can be used include a pinned root, or a side entry fir tree, but those these solutions required side access, and side access limits the steam path design and is more costly.
It is also possible to use a straddle root but this requires a window, i.e. a gap in the location ridge, to enable assembly. Any proposal that requires a window or a modified closing blade necessarily involves a weaker blade root at that point, and this limits the design of the whole blade ring, resulting in lower load carrying performance.